The contact material of vacuum interrupters is required to satisfy characteristics, such as (1) the breaking capacity being large, (2) the withstand voltage capability being high, (3) the contact resistance being low, (4) the deposition resistance property being high, (5) the contact consumption being low, (6) the chopped current being low, (7) the workability being excellent, and (8) the mechanical strength being high.
Since some of these characteristics conflict with each other, there is no contact material satisfying all of the above characteristics. Cu—Cr electrode materials have characteristics, such as the breaking capacity being large, the withstand voltage capability being high, and the deposition resistance property being high. Therefore, they are widely used as contact materials of vacuum interrupters. Furthermore, there is a report that, in Cu—Cr electrode materials, one having a finer particle size of Cr particles is superior in breaking current and contact resistance (for example, Non-patent Publication 1).
In recent years, there has been progress in making vacuum interrupters conducting arc extinction of vacuum circuit breakers have smaller sizes and larger capacities. Thus, there has been an increasing demand for Cu—Cr based contact materials having withstand voltage capabilities superior to those of conventional Cu—Cr electrodes, which are essential for making vacuum interrupters have smaller sizes. Furthermore, the use conditions of vacuum interrupter users have become severe, and the expansion of applying vacuum interrupters to capacitor circuits has been progressing. In capacitor circuits, the voltage that is the double or triple of normal voltage is applied between the electrodes. With this, the contact surface tends to be considerably damaged by arc at the time of the current breaking and the current opening and closing, and reignition of arc tends to occur. Therefore, there is an increasing demand for electrode materials having breaking capabilities and withstand voltage capabilities, superior to those of conventional Cu—Cr electrode materials.
For example, in Patent Publication 1, there is described a method for producing an electrode material, in which, as a Cu—Cr based electrode material excellent in electrical characteristics such as current breaking capability and withstand voltage capability, respective powders of Cu used as a base material, Cr for improving electrical characteristics, and a heat-resistant element (Mo, W, Nb, Ta, V, Zr) for making the Cr particles finer are mixed together, and then the mixed powder is put into a mold, followed by pressure forming and making a sintered body. Specifically, a heat-resistant element, such as Mo, W, Nb, Ta, V or Zr, is added to a Cu—Cr based electrode material containing as a raw material a Cr having a particle size of 200-300 μm, and the Cr is made fine through a fine texture technology, an alloying process of the Cr element and the heat-resistant element is accelerated, the precipitation of fine Cr—X (Cr making a solid solution with the heat-resistant element) particles in the inside of the Cu base material texture is increased, and the Cr particles having a diameter of 20-60 μm in a configuration to have the heat-resistant element in its inside are uniformly dispersed in the Cu base material texture. Furthermore, in Patent Publication 1, there is a description that it is important to increase the content of the Cr or the heat-resistant element in the Cu base material in the Cu based electrode material and to conduct a uniform dispersion after making the particle size of Cr, etc. fine, in order to improve electrical characteristics such as current breaking capability and withstand voltage capability in electrode materials for vacuum interrupters.
Furthermore, in Patent Publication 2, without going through the fine texture technology, a powder obtained by pulverizing a single solid solution that is a reaction product of a heat-resistant element is mixed with a Cu powder, followed by pressure forming and then sintering to produce an electrode material containing Cr and the heat-resistant element in the electrode texture.
However, if the pulverized arc-resistant metal (the heat-resistant element and Cr element) powder and Cu powder are mixed together as described in Patent Publication 2, depending on the mixing proportion of the heat-resistant element and Cr powder, the arc-resistant metal may aggregate in the electrode texture to cause lowering of the withstand voltage property and the breaking capability.
Furthermore, as described in Patent Publication 3, even if electrode materials have the same composition, they become different in breaking characteristic and conductivity, depending on also the particle size distribution of the Cr powder (and the heat-resistant element powder) to be mixed with the Cu powder.